VERTICAL FLUX OF HORIZONTAL MOMENTUM TRANSPORTED BY LARGE AMPLITUDE GWS ABOVE SOUTHERN ANDES, THE DRAKE PASSAGE AND THE ANTARCTIC PENINSULA

DE LA TORRE, A.; ALEXANDER, P.; HIERRO, R.; LLAMEDO, P.; ROLLA, A.; SCHMIDT, T.; WICKERT, J.

Congreso; COSPAR; 2012

We analyze large-amplitude mountain and shear gravity waves above the Andes range to thesouth of the highest tops (around 32S), the Patagonia region and its prolongation in the Antarctic Peninsula, from Weather Research and Forecasting (WRF) mesoscale model simulations during winter 2009. This study was mainly motivated by long-term satellite observations of large amplitude gravity waves in the lower and middle atmosphere as well as the recent design of a research program to investigate these features in detail: the Southern Andes Antarctic Gravity wave Initiative (SAANGRIA). The simulations are forced with ERA-Interim data from the European Centre for Medium-Range Weather Forecasts. From 1 June to 31 August 2009, several case studies were selected on the basis of their outstanding characteristics and large wave amplitudes. In general, one or two prevailing modes of oscillation are identied, after applying continuous wavelet transforms at constant pressure levels and perpendicularly to the nominal orientation of the dominant wave crests. In all cases, the dominant modes are characterized by horizontal wavelengths around 50 km. Their vertical wavelengths, depending on a usually strong background wind shear, are estimated to be between 2 and 11 km. The corresponding intrinsic periods range between 10 and 140 min. The synoptic circulation for each case is described. The calculated zonal and meridional components of the vertical flux of horizontal momentum are shown. Large values of this flux are observed at higher pressure levels, decreasing with increasing height after a progressive deposition of momentum by different mechanisms. As expected, in the wintertime upper troposphere and lower stratosphere in this region, a prevailing zonal component is negative almost everywhere, with the exception of one case above the northern tip of the Antarctic Peninsula. A comparison with previous experimental results reported in the region is performed. Partial wave reflection near the tropopause was found, as considerable departures from equipartition between potential and kinetic wave energy are obtained in all cases and at all pressure levels. This ratio is generally less than 1 below the lower stratosphere.